Critical cutting thickness in ultra-precision machining of single crystal silicon

This paper makes use of a strain gradient theory to obtain excellent consistency between observed experimental phenomena and theoretical calculations in exploring the brittle–ductile transition mechanism of single crystal silicon (SCS). The critical cutting thickness in the ultra-precision machining of SCS is then derived by means of theoretical calculations. SCS was first subjected to nanoindentation, and it was observed that under a particular scale of deformation, the silicon not only underwent plastic deformation, but more importantly also experienced strain gradient effects. This can be attributed to different types of dislocation motion present in the crystal, suggesting that the plastic deformation of SCS is caused by geometrically necessary dislocations, and that a size effect fulfills the necessary conditions for plastic region machining of SCS. Subsequently, the ability of scale gradient theories to link together microscopic mechanisms with observable mechanical properties was utilized to calculate the critical cutting thickness in the ultra-precision machining of SCS as approximately between 110 and 220 nm, a result which was then verified by experimental means.     

Critical dwell time of switched linear systems

In this paper, we consider the relation between the switching dwell time and the stabilization of switched linear control systems. First of all, a concept of critical dwell time is given for switched linear systems without control inputs, and the critical dwell time is taken as an arbitrary given positive constant for a switched linear control systems with controllable switching models. Secondly, when a switched linear system has many stabilizable switching models, the problem of stabilization of the overall system is considered. An on-line feedback control is designed such that the overall system is asymptotically stabilizable under switching laws which depend only on those of uncontrollable subsystems of the switching models. Finally, when a switched system is partially controllable (While some switching models are probably unstabilizable), an on-line feedback control and a cyclic switching strategy are designed such that the overall system is asymptotically stabilizable if all switching models of this uncontrollable subsystems are asymptotically stable. In addition, algorithms for designing switching laws and controls are presented.     

Numerical investigation of the solar concentrating characteristics of 3D CPC and CPC-DC

A Monte Carlo Ray Tracing (MCRT) method was expanded with a bisection module to solve high-order nonlinear equations. Applying this modified MCRT method, the solar concentrating characteristics of a 3D Compound Parabolic Concentrator (3D CPC) were investigated. Moreover, a two-stage solar concentrator formed by a dish concentrator in tandem with a single 3D CPC (CPC-DC) was presented in this paper. Considering the influence of the tracking errors and slope errors, the solar concentrating performances of the CPC-DC was performed using the modified MCRT method. For DC having a rim angle of 45°, the numerical results show that the interception efficiency of the DC is about 4.0% higher than that of CPC-DC, but the concentrator ratio of the CPC-DC is twice as large as that of DC.     

Dynamics and control of solar thermochemical reactors

A general dynamic model for solar-driven thermochemical processes is formulated based on unsteady mass and energy conservation equations coupled to the reaction kinetics. It is applied to two pertinent high-temperature thermochemical reactors for fuel production that make use of concentrated solar energy as the source of process heat, namely: an indirectly irradiated batch-operated packed bed reactor for the carbothermic reduction of zinc oxide, and a directly irradiated continuously operated particle flow reactor for the steam-gasification of petcoke. Model parameter identification and validation is accomplished by comparing numerically simulated and experimentally measured temperatures and outlet product concentrations. A linear feedback controller was implemented using the LQG/LTR design method. Simulations of the controlled reactor system with real solar irradiation data indicates improved quality and steadiness of product composition throughout transient solar input phases and superior solar-to-chemical energy conversion efficiency.     

Ti/Al扩散偶中间相长大过程的数值模拟(英文)

为研究固态Ti/Al扩散偶的扩散反应,将Ti/Al箔构成的扩散偶分别在525,550,575和600°C退火1～40h。实验结果表明TiAl3是Ti/Al界面处生成的唯一相。TiAl3的优先长大是界面热力学作用的结果。TiAl3相主要向Al箔一侧长大,其长大过程符合抛物线规律。在晶界扩散的基础上,用有限差分方法模拟TiAl3相的长大过程,模拟结果和实验结果吻合较好。     

微尺度铸件充型过程的数值模拟

修正了传统Navier-Stoke方程,并利用该方程和Fluent软件对Zn-4Al合金充填微齿轮铸件的充型过程进行数值模拟.结果表明:在运动惯性的作用下,Zn-4Al合金在进入微齿轮型腔后途径齿轮盘部位时,合金并未横向扩展,而是保持入射状态首先填充对面的齿轮轴,撞击型腔壁后,产生二次压头,然后再向齿轮外围的各齿部位反充;型腔内气体被高速运动的金属液搅拌和切割,形成许多微气泡,然后被带入主流区,从排气道排除.     

Deformation and fracture behavior of two Al-Mg-Si alloys

Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.     

Microstructures and wear resistance of large WC particles reinforced surface metal matrix composites produced by plasma melt injection

Large WC particles (− 840 μm-+ 420 μm) reinforced surface metal matrix composites (SMMCs) were produced using plasma melt injection (PMI) process on a Q235 (similar to ASTM A570 Gr. A) low carbon steel substrate. Microstructures of the SMMC were observed using scanning electron microscope (SEM), and the composition was determined with energy dispersion spectroscopy (EDS). Phases were analyzed with X-ray diffraction. Micro-hardness of the SMMC was tested. Wear losses of the SMMC layer were evaluated under dry friction conditions and compared with those of the substrate material. The results show that the large WC particles are caught by crystallized metal and stay in the upper part of the SMMC layer, and there is only a little melting on the outer surface. No sinking down of WC particles occurs. The SMMC layer is well bonded to the substrate, and the interface is crack free. The wear resistance of the Q235 substrate is greatly improved with large WC particles injected.     

Study on the recovery of zinc from Moore cake: a biotechnological approach

To assess the potential for recovering zinc using a biotechnological approach, leaching of Moore cake using Thiobacillus ferrooxidans bacteria has been attempted. At laboratory scale a number of experiments were conducted at different pH values, different ratios of solid to leach-liquor, different temperatures etc. Shaking and percolation effects on leaching were also studied. Maximum zinc recovery was found to be about 40% in a percolation experiment.     

切削加工中的刀—屑接触区应力分布计算模型建立

切削加工中,刀—屑接触区的应力分布直接影响着切削加工性能、切削温度分布及刀具磨损等,本文通过对直角切削时的切削力的分析和刀—屑接触长度的计算,建立了刀屑接触区的正应力与剪应力的分布计算模型,通过该模型可直接得到刀具前刀面的应力分布情况.